Distributed quantum information protocols and networks require the ability to transfer quantum information between remote locations. Such distributed systems generally use photons that can be transmitted through fibers or other media to convey quantum information. However, such transmissions generally experience signal loss and quantum state decoherence effects. For example, an optical signal when transmitted 10 km along a typical optical fiber experiences a 1.9 dB loss, so that a single photon may have a 50% probability of being lost. Loss and decoherence effects on duplicated or redundant quantum states can create noisy entangled states that may reliably convey quantum information over limited distances, but transmissions over large distance generally require quantum repeaters. With quantum repeaters, entanglement can be created across a channel extending a large distance by dividing the channel into sufficiently short segments or links such that noisy entangled states in each link can be purified and then connected via entanglement swapping. The distributed quantum system can then use the resulting entanglements between distant nodes, for example, to teleport quantum information or transmit secret classical information.
Some known approaches to quantum repeaters use single photon states for transmissions and generate entanglement using post-selection based on single-photon detection. See, for example, L. M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, Nature 414, 413 (2001), L. Childress, J. M. Taylor, A. S. Sørensen, and M. D. Lukin, eprint: quant-ph/0410115 (2005), and L. Childress, J. M. Taylor, A. S. Sørensen, and M. D. Lukin, eprint: quant-ph/0502112 (2005). In these schemes, high-fidelity entanglement is created, and subsequent entanglement concentration is needed primarily to compensate for the degrading effect of connecting the imperfect entangled pairs via swapping. However, due to their low success probabilities in the initial entanglement distribution, these protocols have low communication rates.
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